The objective of the research being conducted is a study of regulation of genomic expression during early development of and differentiation in the sea urchin embryo. Particularly, this work concerns characterization of nuclear DNA obtained from developing embryos. We are interested in DNA sequences and single-strandedness specific to cells undergoing differentiation. Work is also being done on the mode of DNA replication and on the effect of 5-bromodeoxy-uridine on DNA synthesis, transcription and differentiation in embryos.